Among oil field chemicals are scale inhibitors, which are used in production wells to stop scaling in the reservoir rock formation matrix and/or in the production lines downhole and at the surface. Scaling not only causes a restriction in pore size in the reservoir rock formation matrix (also known as `formation damage`) and hence reduction in the rate of oil and/or gas production but also blockage of tubular and pipe equipment during surface processing. To overcome this, the production well is subjected to a so called "shut-in" treatment whereby an aqueous composition comprising a scale inhibitor is injected into the production well, usually under pressure, and "squeezed" into the formation and held there. In the squeeze procedure, scale inhibitor is injected several feet radially into the production well where it is retained by adsorption and/or formation of a sparingly soluble precipitate. The inhibitor slowly leaches into the produced water over a period of time and protects the well from scale deposition. The "shut-in" treatment needs to be done regularly e.g. one or more times a year at least if high production rates are to be maintained and constitutes the "down time" when no production takes place. One such method is that described in U.S. Pat. No. 5,002,126 in which a water-soluble surfactant metal salt containing a terminal metal ion is injected down the wellbore and into the reservoir, the surfactant is adsorbed on the surfaces of the reservoir, and thereafter an aqueous solution containing a scale inhibitor capable of reacting with the surfactant is injected into the reservoir so as to form a metal inhibitor which slowly dissolves in water produced from the reservoir. Over the year there is a reduction in total production corresponding to the number of down times during the squeeze/shut-in operation, as well as reduced production as the scaling problem builds up. However, in some instances the scale inhibitor is poorly retained within the reservoir rock formation matrix and short squeeze lifetimes are experienced. The net result in these cases is frequent well interventions which impact on both well productivity and field profitability. One method of alleviating this kind of problem is claimed and described in our prior published WO 96/22451 which relates primarily to adsorption of the scale inhibitor on to the surface of the reservoir rock formation matrix susceptible to scale deposition. In this latter document, the surfactant is so chosen that the retention period of the scale inhibitor on the surface so treated is extended, ie the speed with which the inhibitor is dissolved by produced water is reduced, and consequently the frequency of well intervention is reduced considerably.
Another approach to alleviate the same problem using substantially similar scale squeeze/shut-in technique is to precipitate the scale inhibitor of low water solubility on to the surface susceptible to scale deposition. One such method is described in U.S. Pat. No. 4,357,248. According to this publication, a subterranean reservoir is treated by injecting into it a self-reactive inhibitor solution which subsequently precipitates a scale inhibitor of low water solubility on the relevant surfaces of the reservoir rock formation matrix. In this process, an anionic scale inhibitor and a multivalent cation salt are dissolved in an alkaline aqueous liquid to provide a solution which contains both scale-inhibiting anions and multivalent cations which are mutually soluble as the alkaline pH, but which, at a lower pH and the temperature of the reservoir are precipitated as a scale inhibiting compound having an effective but relatively low water solubility. At least one compound which reacts at a relatively slow rate to reduce the pH of the alkaline solution is also dissolved in the solution. The rate at which the pH of the solution is reduced is adjusted, by arranging the composition and/or concentration of the compounds dissolved in the solution to correlate the rate of pH reduction with the temperature and injectivity properties of the well and reservoir. This is the so called "precipitation squeeze" method.